SctK

SctK is a conserved protein component of bacterial type III secretion systems (T3SS), a syringe-like apparatus used by many Gram-negative pathogens to inject effector proteins into host cells. The SctK protein is typically encoded within T3SS gene clusters, and orthologs are known in several species where it is designated as YscK in Yersinia or PscK in Pseudomonas.

In the cytoplasm, SctK is part of the sorting platform that sits at the cytoplasmic face of the basal body. It contributes to the assembly and function of the platform, which coordinates substrate recognition, chaperone release, and secretion timing. SctK is thought to act as a scaffold or linker that helps assemble and stabilize interactions among other cytosolic components, including SctQ and SctL, and possibly interfaces with the ATPase SctN. The exact interactions and stoichiometry can vary among T3SSs from different bacteria.

Expression of sctK is coordinated with other T3SS genes and is often regulated by environmental cues and regulatory networks that govern secretory activity. Disruption of sctK can impair secretion and attenuate virulence in model infections, underscoring its role in the proper functioning of the secretion system.

SctK exemplifies the diversity and conservation of the T3SS core components, reflecting a modular architecture that supports regulated protein export into host cells.